The present invention relates to a torque detection device for detecting a torque applied to a rotating body.
An exemplary torque detection device is disclosed in Japanese Patent Application Laid-Open No. 2005-69994 (which will be hereinafter simply called “conventional example”), for example. The torque detection device according to the conventional example includes: a magnetic circuit forming member having a tubular magnet provided at either one of input and output shafts connected via a torsion bar, and a magnetic ring that is located circumferentially of the tubular magnet and rotated together with the other one of the input and output shafts; a magnetic flux collecting ring part for collecting a magnetic flux generated by the magnetic circuit forming member; and a detection part for detecting, based on the density of the magnetic flux collected by the magnetic flux collecting ring part, a torque applied to the input shaft or the output shaft.
The torque detection device constructed in this manner is incorporated into an electric power steering apparatus for a vehicle. The electric power steering apparatus includes: an input shaft connected to a steering wheel; a housing for accommodating and supporting the input shaft; an electric motor connected to an output shaft via a deceleration mechanism; and a control unit which is connected to a circuit for driving the electric motor and a detection part, and in which a microprocessor is used. And the apparatus is constructed so that the detection part detects, due to a torsion generated in a torsion bar, a rotational torque applied to the input shaft by the steering of the steering wheel, and the electric motor is driven and controlled based on the detected torque.
The tubular magnet according to the conventional example is provided by sintering ferrite powder into a tubular shape. However, when the sintered tubular magnet is externally fitted and fixed to the input shaft, a crack is likely to occur due to a radial inner pressure generated at an inner circumferential portion of the tubular magnet. To cope with this, according to the conventional example, the tubular magnet is molded with a synthetic resin material in advance, and the molded tubular magnet is externally fitted and fixed to a knurled portion of the input shaft, which is located at its outer circumferential portion, thereby preventing a crack during the assembly of the tubular magnet.